Check the parsed AST in the tests

First of all, thanks for your great project, it helps me very much with parsing microsoft function definitions in their headers.

In spite of the last point of #59 I have some concerns regarding the test suite: it does not seem to check the resultant AST or generated code correctness.

I think it might be worthwhile to add such checks to all the tests. As of now I am considering replacing all the `ast.show()` and `print(GnuCGenerator().visit(ast))` in the tests with checks against known good values.

The problem I see is that `ast.show()` results are incomplete (they lack attributes in some cases, for example), so it might not be the best idea to use it as-is.

What do you think regarding this issue? Maybe there is a better way to approach this issue?